1. Field of the Invention
The present invention relates to a power transmission system for a vehicle changing a power transmission state by electric control to transmit and stop driving force by operating an electromagnet, particularly a technology relating to a power transmission system for a vehicle changing a power transmission state by electric control which can be applied to a coupling, a transfer case, a limited slip differential, and a torque vectoring axle of vehicles.
2. Description of the Related Art
Power transmission system for a vehicle changing a power transmission state by electric control, which is equipped with an electromagnet in the related art, as shown in FIG. 1, mainly use a mechanism in which an armature 73 that is attracted by the magnetic force of a circular electromagnet 79 fixed to a main housing 33 operates a pilot clutch 71 by applying electric current to the electromagnet 79, frictional torque generated by the pilot clutch 71 creates relative rotational velocity between a cam 63 constituting a ball ramp and an apply plate 61 to generate axial magnified-force between the cam 63 and the apply plate 61, the apply plate 61 presses a main clutch 41, and rotational force of an input shaft 1261 is transmitted to an output shaft 51 through the main clutch 41.
The armature 73 should be positioned opposite to the electromagnet 79 across the pilot clutch 71 to operate the pilot clutch 71 using the magnetic force of the electromagnet 79. Further, the pilot clutch 71 should be made of steel through which lines of magnetic force can easily pass, and should have a long slot for paths of the lines of magnetic force and a coil housing 39 between the pilot clutch 71 and the electromagnet 79 should have a structure made of different materials, which are welded, such as stainless steel that is a non-magnetic material in order to create a closed loop of the lines of magnetic force for smoothly supplying the magnetic force of the electromagnet 79 to the armature 73.
For reference, FIG. 2 shows a graph comparing forces (attracting force) exerted in the armature according to the amount of electric current applied to the electromagnet, when a different material, which is a non-magnetic material, such as stainless steel, is welded to the coil housing and when the coil housing is manufactured only by steel of one material without welding a different material, in which it can be seen that significantly large force is exerted in the armature for the same electric current, when the coil housing where a different material, such as stainless steel, is welded is used.
Meanwhile, it is difficult to make the main clutch 41 and the pilot clutch 71 of the same material, because the main clutch 41 should be made of a material other than steel to transmit large driving force.
Further, since the coil housing 39 has a structure made of different materials, which are welded, such as stainless steel 89, as described above, the manufacturing is difficult and the cost correspondingly increases.
On the other hand, since a gap from a magnetic coil of the electromagnet 79 to the armature 73 is a very important and sensitive factor that determines the operational force of the armature 73 by the lines of magnetic force and should be precisely maintained, a difficult process of adjusting an error of machining in manufacturing, using a shim, is needed.